This invention relates to fuses in general and in particular to an electric fuse that meets the minimum requirements of the Underwriter's Laboratories (UL) specification for Class J dimensioned fuses having time delay. A time-delay fuse is a type of fuse that has a built in delay that allows temporary and harmless inrush currents to pass without opening, but is designed to open on sustained overloads and short circuits.
The time-delay fuse can be a dual-element fuse and is used in circuits subjected to temporary inrush current transients, such as motor starting currents, to provide both high performance short-circuit current protection and time-delay overload current protection. Over sizing in order to prevent nuisance openings is not necessary. The dual-element fuse contains two distinctly separate types of elements which are series connected. Fuse links similar to those used in the single-element fuse perform the short-circuit protection function. The overload element provides protection against low-level over currents or overloads and will hold a overload which is five times greater than the ampere rating of the fuse for a minimum time of 10 seconds.
Underwriter's Laboratories has developed basic physical specifications and electrical performance requirements for fuses with voltage ratings of 600 volts or less. These are known as UL Standards. If a type of fuse meets the requirements of a standard, it will be placed in that UL Class. Typical UL Classes are K, RK1, RK5, G, L, H, T, CC, and J.
Class J fuses are rated to interrupt 200,000 amperes a.c. They are UL labeled as "Current Limiting", are rated for 600 volts a.c., and are not interchangeable with other classes. In order for a Class J fuse to be a time-delay fuse it is necessary that the fuse meet not only the voltage and current characteristics required but the physical size limitations required by Underwriter's Laboratories. Thus the time-delay element and the short circuit element must be small and compact. It is necessary to have a fuse which is high capacity, fast acting, with time-delay, and yet will fit in the small package dictated by Underwriter's Laboratories for Class J fuses.